In a cell-divided mobile radio system, the geographic regions of the system are divided into cells which are collected into larger groups, often referred to as clusters. Each cell is allocated a number of carrier frequencies in accordance with a frequency plan, so that mutually adjacent cells will not disturb one another and so that a carrier frequency pattern can be formed in the group. Each cell may have a base station, and it is also possible to serve several cells from a single base station. The pattern of carrier frequencies is then repeated with each group. A mobile radio system of this kind is described in more detail in CMS88, Cellular Mobile Telephone System, Ericsson Telecom, 1988, Chapter 6.
In an adaptive mobile radio system, each base station is able to transmit on all carrier frequencies and the channels are allocated to a mobile station subsequent to having established the extent to which the separate carrier frequencies are disturbed. As the channels are allocated, there forms a changing geographic pattern of carrier frequencies which are so allocated that no one connection will disturb the other. Channel allocation is normally effected with the aid of a control channel on a carrier frequency, and traffic signals are transmitted on traffic channels of other carrier frequencies. When the number of carrier frequencies is limited, it is extremely difficult to avoid disturbances between separate connections which use one and the same carrier frequency. Among other things, this difficulty is because the mobile stations are liable to have a disturbing effect of varying magnitude when they move. Various methods have been proposed for creating a measurement of the quality of a connection, for instance with the aid of SAT (Supervising Audio Tone) as described in the aforesaid reference CMS88, Chapter 1:10. The base station transmits on a desired carrier frequency a tone which has a modulation frequency above the audible range. The tone is received by the mobile station and retransmitted to the base station. The base station can determine the extent to which the selected carrier frequency is disturbed, by comparing the transmitted tone with the retransmitted tone. In the case of digital transmission systems, the Bit Error Rate, BER, can be used as a measurement of the disturbance. The drawback with both of these methods, however, is that they give an indirect measurement of how the carrier frequency from a disturbing connection influences the home connection.
In a thesis submitted at the Royal Institute of Technology (Kungl. Tekniska Hogskolan) by Christer Gustavsson and entitled "Simulering av adaptive kanalval" (Simulation of Adaptive Channel Selection), Ericsson Radio Systems AB, 1987, there is described a method of reducing the number of calls which are blocked when changing base stations, for instance. The base stations have, in the main, permanently allocated channels, but they also have a smaller number of adaptive channels. The base station and the mobile station measure the signal strength on the call channels of the system and use these measurements as a measurement of the signal strength of traffic channels which can be connected. The mobile station and the base station also measure the signal strength of traffic channels between foreign mobile stations and base stations. These traffic channels constitute disturbances on the connection to be established. There is selected a traffic channel whose disturbance has a low signal strength in relation to the signal strength of the call channel. Although the method enables the transmission quality of a traffic connection to be adequately assessed and also provides for an adequate choice of traffic channel, it does not provide a direct measurement of the signal strength of the selected traffic channel, which is a disadvantage. Neither does the method provide the possibility of measuring continuously the transmission quality of the home traffic connection that has been established. A similar method of determining transmission quality has been proposed for the European mobile telephony system. This method is described in ETSI/GSM 05.08, Version 3.5.0, Chapter 3.